MEPS 566:229-241 (2017)  -  DOI: https://doi.org/10.3354/meps12024

ABSTRACT: To reduce interspecific competition, sympatric species must segregate their resources in a variety of dimensions. Otariid seals (fur seals and sea lions) breed sympatrically in several regions and, where this occurs, differences in lactation length and body size (which influence foraging behaviour and diet) are apparent. However, congeneric Australian fur seals Arctocephalus pusillus doriferus (AUFS) and New Zealand fur seals A. forsteri (NZFS) breed sympatrically on several islands within south-eastern Australia, and display complete overlap in breeding period. How these populations segregate resources is unknown. We assessed the foraging ecology and diet of adult females of both species breeding on Kanowna Island, south-eastern Australia. Foraging locations and diving behaviour differed between species, with AUFS diving deeper (consistent with benthic foraging; 70.6 ± 2.3 m [SD]), while NZFS predominantly dived to shallow depths (16.9 ± 3.7 m), suggesting an epipelagic foraging mode. A bimodal pattern in foraging range was observed in NZFS, with animals either foraging near the colony (15.7 ± 13.0 km) or travelling beyond the continental shelf (363.4 ± 17.2 km), while AUFS foraged within 79.8 ± 8.8 km of the colony. Although dietary composition was similar, the relative importance of prey differed. NZFS predominantly consumed pelagic species, while AUFS primarily consumed a variety of benthic/demersal species (niche overlap 0.39). These differences coincide with the divergence in population demography of the 2 species (AUFS exhibit lower, more stable fecundity compared to NZFS) and are consistent with predictions that foraging mode influences life history traits in otariid seals.

KEY WORDS: Benthic diving · Pelagic diving · Spatial distribution · Diet overlap · Australian fur seal · New Zealand fur seal